Empirically is the only way to know, if you are cleaning based on anything other than actually testing each rifle for what it wants in it's cleaning you are doing it "wrong". You either want to clean for accuracy or consistent velocity depending on your use. This will vary by caliber, barrel, load, etc. Practical guns get cleaned different than benchrest guns, get cleaned different than hunting guns etc. Cleaning a gun is different than changing your oil, because you can wear/damage a barrel cleaning it wrong/too often. It's just like using the right cleaning methods/tools will minimise barrel wear from cleaning, and using the wrong ones can damage a barrel in a single cleaning session. IMO most guys spend too much effort cleaning copper out, and not enough on carbon and most shooters would be much better off just doing chamber/throat/carbon cleaning after range sessions, and only doing copper cleaning at much less frequent intervals.
A borescope is cute, but be prepared to be horrified at what you see, I've seen factory barrels look so bad the average person would not want to shoot them that shot great. I can show pictures from a borescope in a custom S&W 41 barrel that look so bad the owner returned it. That barrel was then tested and yielded 10 shot sub 0.5" groups at 50 yards with eley match. A well known aftermarket AR barrel maker's new pitted barrel that shot lights out. To me they are really only useful for watching how a barrel carbon/copper fowls to determine cleaning intervals/methods, and monitoring throat/chamber/crown erosion.
I've seen more damage done to guns that were over cleaned, and cleaned incorrectly than I ever have from under cleaned. I've seen .308 custom barrels on loaner guns that were not cleaned for thousands of rounds, and were so bad the first few inches of rifling was shot out, but the barrel still shot sub 1MOA, and I've seen factory barrels so bad that after 20 rounds it looked like the barrel was made from copper, and groups went from 1" to 3". Which is why you get 100 different answers from 100 different shooters that all seem to work for them.